Can't add iCloud id
macOS High Sierra (10.13.5)
If you are trying to add your iCloud email address, you don't need to. My iCloud email address doesn't show in System Preferences/Internet Accounts and it works normally.
When I move my three fingers in the upward direction on the trackpad and it highlights say google or icloud or directly tap the desired one it shows , but the query is when I click on plus sign the mail address can't be added .
You are signed because it shows iCloud at the top of the list in your screen shot.
Can't add iCloud id